Chromium containing coatings are applied to aluminum substrates to provide corrosion protection and to improve the adhesion of organic coatings to the aluminum substrate.
The methods for applying chromium containing coatings to aluminum substrates were developed and brought to commercialization in the early forties. The early processes are disclosed in publications such as U.S. Pat. No. 2,438,877, U.S. Pat. No. 2,494,910, U.S. Pat. No. 2,678,291, and U.S. Pat. No. 2,859,147. These processes are still used today as can be seen from U.S. Pat. No. 4,668,305 which is an improvement in the earlier processes. The earliest processes were concerned with applying a chromium phosphate containing coating to an aluminum substrate.
Another commercial process for applying a chromium containing coating to an aluminum substrate was developed in the early 1950's and did not require the presence of a phosphate ion in the coating bath. Processes of this type are disclosed in U.S. Pat. No. 2,796,370, U.S. Pat. No. 2,796,371, U.S. Pat. No. 2,843,513, and U.S. Pat. No. 2,859,144. An improvement in these process was patented as late as 1979 as evidence by U.S. Pat. No. 4,146,410. The disclosures of the chromium phosphate containing coating and chromium oxide containing coating process patents are incorporated herein by reference.
All of these processes are known to provide coatings which improve the corrosion resistance and improve the organic coating adhesion of aluminum substrates.